Central line trains, unlike most other lines, do not have on-board cameras in the carriages, the Assembly was told.

But TfL has since revealed that it was bringing forward its overall £350m upgrade programme starting next year.

A TfL spokesman said later: "This is a bit of a red herring. Most sexual offences such as groping are in crowded carriages are in the rush-hour and are difficult to spot.

"The CCTV in the station can be more valuable for identifying offenders in these instances, compared to onboard CCTV, as it provides a clearer picture. That's where we catch offenders."

Central line... more incidents than any other part of the London Underground. Picture: Mike Brooke

Transport police have 3,000 officers running covert patrols on the Underground, it has emerged.

TfL's director of policing Siwan Hayward revealed: "More undercover patrols are on the Central line than any other line. A programme of work is underway to install CCTV on the line as quickly as possible from 2020."

An increase in victims reporting unwanted sexual molesting has led more than 1,000 suspects being arrested.

The Assembly urges TfL to promote awareness of the limited period for public transport CCTV images to be retained.